Sizing is the process of providing cellulosic articles such as, for example, paper and paper board with resistance to penetration by liquids. Sizing may be accomplished via an internal sizing process, an external sizing process, or as in the usual case, a combination of both.
The processes of internal sizing and external sizing are very different from each other in many respects. For internal sizing, sizing is initiated before the cellulosic article is completely formed. Internal sizing usually is accomplished by adding an internal sizing agent, in conjunction with a retention aid, directly to an aqueous pulp slurry wherein the sizing agent coats the fibers of the pulp. Internal sizing agents are generally hydrophobic in nature wherein their nonpolar portions are anchored to the surface of fibers and thereby retard water penetration when the fibers are completely formed or fabricated into the finished cellulosic article. See, Biermann, C. J., Essentials of Pulping and Papermaking, Academic Press, Inc., 1993, p.197.
External sizing is also referred to in the art as surface, tub, or calender sizing. For external sizing, sizing agents are applied to one or both surfaces of a completely formed cellulosic article, generally without the addition of retention aids. In contrast to the hydrophobic materials required for internal sizing, non-hydrophobic materials, such as, starch, are commonly used as external sizing materials. Whereas, internal sizing takes place on surface of pulp fibers in a slurry, external sizing occurs when an external sizing material is applied to the surface of a fabricated cellulosic article and fills the capillaries of the article, rendering water penetration more difficult.
For internal sizing of paper, Vaughn et al. in U.S. Pat. Nos. 3,872,039; 3,899,389; and 4,181,566, described the combination of an ammoniated copolymer of ethylene and an ethylenically unsaturated carboxylic acid and a cationic retention aid. The advantage of the sizing systems disclosed by Vaughn et al was said to be their utility over the complete range of pH conditions found in paper-making operations. For these systems, Vaughn et al. taught the order of addition of the sizing agent and the retention aid to a conventional paper making process was not critical. Nevertheless, Vaughn et al. preferred to add the sizing agent prior to the addition of the retention aid.
Rowland et al. in U.S. Pat. Nos. 5,206,279 and 5,387,635 disclosed aqueous dispersions of copolymer of ethylene and an ethylenically unsaturated carboxylic acid. Rowland et al. described the combination of two bases, one as a weak cation and the other as a strong cation, that was said to permit the preparation of stable dispersions of copolymers having relatively low carboxylic acid concentrations (e.g. less than 15 weight percent acrylic acid content).
While there are variety of known sizing systems, there is still a need for a wet-end internal sizing system that provides improved cellulosic sizing performance, i.e., higher Hercules Size Test (HST) values at 80 percent reflectance as measured in accordance with TAPPI method T 530 pm-89. Higher HST values translate into improved hydrophobicity and water penetration resistance. There is also a need for a sizing system that permits the use of various retention aids. For example, sizing operators desire the versatility of being able to use retention aids that provide good wet strength (e.g., Kymene 557) on some occasions and, on other occasions, employing retention aids that do not provide good wet strength (e.g., Nalco 7583). There is also the need to provide a sizing system wherein the sizing agent is stable and resistant to precipitation from hard water dilutions and yet is easily repulpable when, for example, operators desire cellulosic articles and substrates with enhanced wetting characteristics. There is also the need to provide a sizing system that has improved receptivity to various fillers without disturbing the basic sizing performance of the system.